1. Field of the Invention
The present invention relates in general to dense wavelength division multiplexing (DWDM) systems and, in particular, to a system and method for measuring wavelength errors of one or more signals.
2. Description of Related Art
Today, the inaccuracy and long term stability of wavelength lockers imposes one of the most significant limitations to the capacity achievable in a DWDM transmission system. Wavelength lockers are used to identify and correct wavelength errors in optical signals emitted from lasers. An exemplary Dense WDM (DWDM) system 100 incorporating traditional wavelength lockers 102 and the problems associated with the traditional wavelength lockers 102 are described below in FIG. 1.
Referring to FIG. 1 (PRIOR ART), there is a block diagram of the exemplary DWDM system 100 in which the traditional wavelength lockers 102 (only four shown) are not incorporated within a traditional wavelength combiner 104. The DWDM system 100 includes an output unit 106 (e.g., optical source 106) and an input unit 108. The output unit 106 includes one or more transmitters 110 (only four shown) that are connected to the wavelength combiner 104 (e.g., wavelength multiplexer 104). The input unit 108 includes one or more receivers 112 (only four shown) that are connected to a wavelength splitter 114 (e.g., wavelength demultiplexer 114). As shown, the transmitters 110 are connected to the wavelength combiner 104 which is connected via an optical fiber 120 to the wavelength splitter 114 that is connected to the receivers 112. Each receiver 112 includes an O/E device 118 (e.g., PIN or APD) and a CDR 120 that demodulates and outputs transmitted data (e.g., channel 1 data, channel 2 data . . . ).
Each transmitter 110 includes a laser 122, an internal or external modulator 124 (shown as an external modulator 124), a data source 126, a traditional wavelength locker 102, a feedback circuit 128 and a thermoelectric cooler (TEC) 130. As shown, each laser 122 (e.g., thermally tunable laser 122) is connected to an external modulator 124 (e.g., Mach-Zehnder modulator, electro-absorptive modulator). The external modulator 124 is connected to the data source 126 (e.g., channel 1 data, channel 2 data . . . ). Each laser 122 and external modulator 124 emit a modulated optical signal 132 (shown as λ1, λ2, λ3 and λn) towards one of the traditional wavelength lockers 102.
Each traditional wavelength locker 102 includes a splitter 134 (e.g., 95/5 splitter 134) that directs a large portion of the modulated optical signal 132 to the wavelength combiner 104 and a smaller portion of the modulated optical signal 132 to another splitter 136 (e.g., 50/50 splitter 136). The splitter 136 then directs a portion of the modulated optical signal 132 to an Etalon 138 and another portion or unfiltered version of the modulated optical signal 132 to photodetector 140a (e.g., PIN diode 140a). The Etalon 138 directs a filtered version of the modulated optical signal 132 to photodetector 140b (e.g., PIN diode 140b). The two photodetectors 140a and 140b output two electrical signals 142 to the feedback circuit 128. The feedback circuit 128 analyzes these electrical signals 142 that indicate whether or not there is a wavelength error and the magnitude of the wavelength error in the corresponding optical signal 132. If there is a wavelength error in the optical signal 132, then the feedback circuit 128 instructs the corresponding thermoelectric cooler 130 (e.g., Peltier Cooler) to adjust the corresponding laser 122 in order to correct the wavelength error in that optical signal 132.
The DWDM system 100 and the traditional wavelength lockers 102 described above are well known to those skilled in the art. Likewise, the problems associated with the traditional wavelength lockers 102 are also well known to those skilled in the art. The main problems associated with traditional wavelength lockers 102 are often attributable to the aging of the splitters 134 and 136, the Etalon 138 and the photodetectors 140a and 140b. The aging of these different components adversely affects the accuracy and sensitivity of traditional wavelength lockers 102 especially because any drift in accuracy will be largely independent from any drift occurring within the wavelength combiner 104. Thus, there is a need for a new design of a wavelength locker that can address the aforementioned shortcomings and other shortcomings of the traditional wavelength lockers 102. This need and other needs are addressed by the integrated wavelength combiner/wavelength locker of the present invention.